Fly larvae, earthworms and duckweed as feeds for
frogs in an integrated farming system

Phounvisouk Latsamy and T R
Preston*

Abstract

Frogs, purchased as 20 day-old fingerlings, were raised in ponds
and, after a period of adaptation, were submitted to 4 treatments
arranged as a 2*2 factorial with 3 replications using a completely
randomized design. The factors were: addition or not of duckweed to
the feed; and earthworms or fly larvae as the main diet. At the
start of the trial the frogs weighed on average 19 g and were fed
the experimental diets for 90 days.

Growth rates and feed conversion (for DM and crude protein) were
better, and mortality was lower, when frogs were fed earthworms
rather than larvae and when they had access to fresh duckweed mixed
with the larvae/earthworms. The net increase in live weight (252 g
in 90 days) on the best diet (earthworms plus duckweed) was better
than in one report concerning frogs fed an artificial diet (200 g
in 120 days).

Comparisons with another aquatic species (catfish) indicated
broadly similar results for growth rate and feed
conversion.

Key words: Aquaculture, feed conversion, growth

Introduction

Frog culture is an important economic activity in Thailand with
high demand for the product in foreign markets such as Malaysia,
Singapore, Hong Kong, Japan, Germany and France (Akasay, 1994).
There are also recent developments in Brazil, where frog meat is
marketed in the form of entire carcasses or of frozen thighs (frog
legs). (Moura and Ramos No Date)

In Laos there is an increasing demand for frogs for consumption
in family households and local markets. There are restaurants in
Vientiane that buy frogs on a regular basis for their customers
(Bounsong 2001).

From hatching of the eggs to market size, it takes 3 to 4 months
which is similar to other commercial aquatic species such as
catfish and tilapia. Frogs can be raised in most locations, as they
require a small area and lower quantity of water than other aquatic
species. However, in Laos the cultivation of frogs is still not
economically attractive because of high operational costs, chiefly
of purchased feed. This is usually imported from Thailand and
contains expensive ingredients (Table 1) (LARReC 2001).
The
proximate analysis of the feed from one feed manufacturer is shown
(Table 2).

Table 1:
Typical composition of concentrate for feeding
to frogs (imported from Thailand)

%

Fishmeal

48.7

Dextrin

10.9

Alfa starch

10

Rice bran

16.3

Vegetable oil

0.6

Tuna oil

2.25

Pig oil

8.95

Vitamins and minerals

2

Vitamin C

0.1

BHT

0.02

Choline chloride

0.2

Table 2.
Proximate composition of concentrate for frogs (%)

Protein

35

Fat

4

Fibre

5

Moisture

12

The common lowland frog (Ranarugulosa) is the
most popular cultured frog in Laos and Thailand. Because the frog
can take in oxygen by the lungs and through the skin, it can be
housed at extremely high density (30 frogs/m2). The
frogs are mainly carnivorous and prefer to eat creatures that are
alive and moving; however, there are reports (Anon1 No
date) that they can be trained to consume "inert" food such as
pelleted feeds.

There appears to be potential for the incorporation of frog
culture in systems of recycling with duckweed being part of the
feed for the frogs (Figure 1).

Figure 1. Frogs
consuming duckweed in a recycling system (Anon2, No date)

According to Akasay (1994), potential sources of "natural" feeds
that could be cultivated as feed for frogs are fly larvae,
earthworms, termites, duckweeds and snails. Earthworms are
appropriate elements in systems of recycling live stock manure
(Bay, 2002), as they are more suitable than biodigesters when the
manure is derived from rabbits and goats (Preston and
Rodríguez, No date). Earthworms are high in protein and would
thus appear to be suitable components in a feeding system for
frogs.

Fly larvae have been recommended as feed for frogs by Sheppard
(No date), as they are rich in both fat and protein (Khan et
al 1999).

The objectives of the study were to compare the growth of frogs
fed with fly larvae or earth worms as part of an integrated farming
system. The hypothesis was that frogs can grow at acceptable rates
when their diet is composed primarily of earthworms or fly larvae.

Materials and methods

Location

The experiment was carried out in the Living Aquatic Resource
Research Centre in Naong Thang village, Vientiane province from
April to October 2006.

Experimental treatments

Frogs were raised in ponds and submitted to 4 treatments
arranged as a 2*2 factorial with 3 replications using a completely
randomized design. The factors were:

Addition of duckweed to the feed

D: Duckweed

ND: No duckweed

Source of protein supplement

EW: Earthworms

FL: Fly larvae

Design of the ponds

The ponds (n = 12) for the frogs were 1*1m in area and 0.5m
deep. They were lined with polyethylene sheet which extended some
0.5m from the edges (Photo 1). The plastic sheet was secured by
sticks at the four corners and each edge of the sheet was wrapped
around a piece of bamboo in order to tighten the sheet. Four bamboo
poles each 1.2 m high were erected at the four corners of each pond
and a plastic net was placed around the poles to make a fence for
the pond. The lower edge of the net was then secured to the surface
with gravel. The total area including the pond was 2*2m. A sheet
of dark plastic mesh was attached to the tops of the bamboo poles
(Photo 2). All the ponds were fertilized with biodigester effluent
at the rate of 250 ml/m2/week. When duckweed was seen to
grow naturally in the ponds it was removed.

Photo 1. Ponds for the frogs

Photo 2. Plastic nets separating the ponds .

Experimental feeds

Duckweed

The duckweed was cultivated in separate ponds and fertilized
with biodigester effluent following the recommendation of Rodriguez
and Preston (No date).

Earthworms

The earthworms were collected from soil in areas adjacent to the
channels used to transport the pig manure from the pens. Only the
large worms were collected, while the small ones were left to grow
in the soil. Laboratory analysis at the National University of Laos
identified the worms as belonging to two species:
Microchaetidae and Monilgastridae. The average
weights and lengths were 0.6 to 1.2 g and 6 to 10 cm for
Microchaetidae, and 0.8 to 1.6 g and 15 to 30 cm for
Moniligastridae.

Larvae

The larvae were produced using as substrate the combination of
pig manure and fermented fish waste, which gave the highest yield
of larvae in Experiment 1. Fifteen enclosures were used in rotation
so that each day the larvae from three enclosures were harvested,
following a 6 day cycle of production as practiced in Experiment 1.

Origin, management and feeding of the
frogs

The frogs (Ranarugulosa) were obtained as 21
day-old fingerlings (n = 600) from a commercial farm in Champasak
province. They were a crossbreed between local females and
"improved" males the parent stock of which had been imported
originally from Europe. In the commercial farm they had been fed on
commercial concentrate feed. On arrival at the Aquatic Centre they
were kept in a nursery pond for one month in order to let them grow
to an adequate size. During this time they were fed a mixture of
commercial feed and fresh larvae, in proportions in succeeding
weeks of 70:30, 50:50, and 30:70 gradually changing to 100% larvae
at the end of the month. For the experiment, 480 of the strongest
frogs (360 females and 120 males) were selected and allocated to
the 12 experimental ponds. At this time they had an average weight
of 20 g and an average length of 8 cm. After one week, the smaller
females were removed in order to maintain 30 frogs per pond; among
which 20 were females and 10 were males.

The frogs were fed twice per day at 06.00 h and 17.00 h. For the
"D" treatment, fresh duckweed was mixed with either fresh larvae or
fresh earthworms in proportions of 30% of duckweed and 70%
earthworms (or larvae) (fresh basis). The quantities were
controlled according to the appetite of the frogs with the
objective of avoiding feed residues. Prior to mixing the larvae
with the duckweed the larvae were suspended in water for 30 minutes
to reduce their motility, as it was observed that otherwise they
migrated from the duckweed into the water, where they were consumed
by the frogs, which then did not consume the duckweed. On the "ND"
treatment, the larvae or earthworms were offered as separate feeds.
The experimental feeds were given over a period of 90 days. The
water in the ponds was replaced daily at between 05.00 and 06.00h.

Measurements

The amounts of feeds offered were recorded daily. At the start
of the experiment, two frogs (one male and female) were selected in
each treatment / replicate and identified with a colored string
placed around the leg of the frog. These "tagged" frogs were then
weighed every 2 weeks during the 90 days of the experiment. Samples
of the feeds were analysed once at the beginning of the experiment
for dry matter (DM) and crude protein in the case of the duckweed
and for DM, crude protein and fat for the earthworms and the
larvae. Samples of water from each pond were taken weekly
immediately after changing the water, for measurements of
temperature, pH and dissolved oxygen. All the chemical analyses
were done according to the procedures in AOAC (1990).

Statistical analysis

The growth rates of the frogs were calculated from the linear
regression of body weight (Y) on days (X) from the beginning of the
experiment. Results for growth rate and feed conversion were
analysed by the General Linear Model in the ANOVA option of the
software in Minitab 2002 (version 3.1). Sources of variation were:
protein source, duckweed, interaction protein source*duckweed and
error.

Results

Water quality

The range in measurements of water quality during the experiment
(dissolved oxygen 6.5 to 8.2 ppm, pH 6.5 to 8.2 and water
temperature 20.0 to 26.0 ºC) indicated that these parameters
were within the range recommended for raising frogs (Uodone
2004).

Chemical composition of dietary
ingredients

The larvae were much richer in fat but with slightly less
protein than for the earthworms (Table 3).

Table 3.
Composition of the feeds given to the frogs

Larvae

Earthworms

Duckweed

DM, % fresh
basis

22.1

21.1

5.33

CP in DM, %

49.3

55.1

38.5

Fat, % in DM

31.3

3.26

nd

nd: Not
determined

Similar data for fly
larvae were reported by Khan et al (1999). (42% crude
protein and 35% ether extract, DM basis). The crude protein content
of the earthworms was similar to that reported by Nguyen Duy Quynh
Tram et al (2005) (59% in DM) but the fat content was lower than
the 7.9% in DM that was reported by these authors. Bay (2002)
reported a crude protein content of 60% and fat of 7.5% in DM. The
crude protein content of the duckweed was close to the maximum
level of 40% in DM, reported in the review by Leng (1999). On a DM
basis the contribution of the duckweed to the diet was less than
10% (Table 4).

Table 4.
Composition of the diets given to the frogs (L larvae; D duckweed,
ND no duckweed, E earthworms)

L-D

E-D

L-ND

E-ND

Fresh basis, %

Larvae/earthworms

70

70

100

100

Duckweed

30

30

0

0

Dry basis, %

Larvae/earthworms

94.1

90.2

100

100

Duckweed

5.94

9.76

0

0

DM of diet, %

17.1

16.4

22.1

21.1

CP in diet DM, %

48.6

53.5

49.3

55.1

Growth performance

Intakes of DM and crude protein tended to be higher when the
frogs received duckweed (Tables 5 and 6) (P=0.06 and 0.12), and
when they were fed earthworms rather than larvae (P=0.097 and
0.048).

Table 6.
Mean values (individual treatments) for live weights, feed intake
and feed conversion of
frogs fed larvae or earth worms with or
without duckweed (90 day growth period)

Larvae

Earth worms

SEM

Prob

No DW

DW

No DW

DW

Live weight, g

Initial

19

19

19

19

0.45

Fina

105a

129a

214b

271c

7.24

0.001

Daily gain

0.892a

1.14a

2.10b

2.84c

0.071

0.001

Feed intake, g/d

DM

3.58

4.05

4.00

4.13

0.137

0.08

CP

1.76a

1.97a

2.20b

2.24b

0.069

0.001

Feed conversion, g
feed / g weight gain

DM

4.02a

3.56a

1.91b

1.46b

0.170

0.001

CP

1.98a

1.73a

1.05b

0.79b

0.085

0.001

abc
Means within rows without common letter are different at P=0.05

However, growth rates and feed conversion (for DM and crude
protein) were better when the frogs were fed earthworms rather than
larvae and when they had access to fresh duckweed mixed with the
larvae/earthworms (Tables 5 and 6 and Figures 2 and 3).

The net increase in live weight (252 g in 90 days) on the best
diet (earthworms plus duckweed) was greater than for frogs fed an
artificial diet containing fish meal, dextrin, starch, rice bran,
vegetable oil, tuna oil, vitamins, minerals, BHT and Choline
chloride (200 g in 120 days) (Bounsong 2001). The growth rate of
frogs raised with pelleted feed and house fly larvae in Mexico
(Rodriquez-Serna et al1996) were reported to be 0.63
g/day which is also considerably less than recorded for the house
fly larvae diets in our experiment (0.89 g/day for larvae alone and
1.14 g/day for larvae plus duckweed).

Mortality

Mortality was lower when the frogs ate earthworms rather than
larvae and there was a strong indication (P=0.059) that it was less
when duckweed was included in the feed (Table 7; Figure 4). The
fact that mortality was higher on the larval diet is in agreement
with the poorer growth rates and could be interpreted as being due
to the fly larvae being nutritionally inferior to the earthworms
for raising frogs.

Table 7.
Mean values (main effects) for mortality of frogs fed earthworms or
fly larvae and with or without duckweed (30 frogs per pond)

Discussion

In the present study, the growth rates of the frogs on the diets
with earthworms and duckweed (2.84 g/day) were much higher than was
reported for Catfish (0.69 g/day) given diets containing rice bran
(60%), trash fish (10%) and earthworms (30%) (Nguyen Duy Quynh Tram
et al., 2005). The feed conversion rates for the frogs (1.46
and 0.79 for DM and CP) were better than was reported for DM
conversion for the catfish (1.58) and poorer for crude protein
conversion (0.46 for catfish). Support for the high nutritive value
of the earthworms for frogs is the finding in the study of Nguyen
Duy Quynh Tram et al (2005) that the growth rates of the Catfish
increased from 0.39 to 0.69 g/day when earthworms replaced 75% of
the trash fish in the diet.

It is difficult to explain the much higher growth rates on the
diets with earthworms, and the lower mortality, compared with the
diets based on fly larvae. The earthworms contained slightly more
protein but very much less fat than the larvae, and for both feeds,
the protein was considerably above the recommended levels for frog
growth. According to Somsueb and Boonyaratpalin (2001), the optimum
protein content of diets for raising frogs intensively is 37% in
DM. This is much lower than the protein level provided by the diets
containing either the larvae or the earthworms in the present
experiment (49 and 54%, respectively). It was observed that the
frogs ate the earthworms more quickly than they ate the larvae;
however, the overall intakes of DM and crude protein were only
slightly higher for the earthworm diets. The other element
affecting the nutritive value of the larvae may have been the high
fat content (31%). Frogs contain very little fat in the carcass,
most being stored as a "fat body" attached to the gonads (Zancanaro et al 1999). It is not known if frogs can digest a diet
with 30% fat. The fact that commercial feeds for frogs contain only
4% fat suggests that fat may not be well utilized.

There appear to be major differences in the content of the first
limiting essential amino acids between earthworms and fly larvae
(Table 8).

The proportions of methionine + cystine and of threonine
in earthworms (relative to lysine = 100) are close to the
recommended proportions, as in the" ideal" protein. By contrast,
BSF and especially House Fly larvae appear to be poorer in
methionine + cystine and BSF very poor in threonine. There are
reports that earthworms accumulate and concentrate methionine found
in the ecosystem in proportions greater than for other amino acids
(Pokarzhevskii, et al 1997). As a feed supplement,
earthworms have also been found to equal or surpass fish meal and
meat meal as an animal protein source for poultry (Edwards 1998),
which is similar to the finding of Nguyen Duy Quynh Tram et al
(2005) with Catfish.

Further research is required in order to corroborate the
superior feeding value of earthworms compared with fly larvae for
raising of frogs.

Conclusions and recommendations

Growth rates and feed conversion (for DM and crude protein) were
better, and mortality was lower, when frogs were fed earthworms
rather than larvae and when they had access to fresh duckweed mixed
with the larvae/earthworms.

The net increase in live weight (252 g in 90 days) on the best
diet (earthworms plus duckweed) was better than in one report
concerning frogs fed an artificial diet (200 g in 120
days).

Comparisons with another aquatic species (catfish) indicated
broadly similar results for growth rate and feed
conversion.

Acknowledgements

The authors wish to acknowledge the Swedish International
Development Authority (Sida) for financial support of this study. I
also would like to thank the Living Aquatic Recourse Research
Center, pig station of the National Institute of Agriculture and
Forestry, for allowing me to use their facilities, and in
particular Miss. Nang, Mr. Touy and Mr. Kamphon for help in the
experiment.

Bay N V 2002 Study of production and utilization of
Earthworms (Perionyx excavatus) as feed supplement in Chicken diet in order to improve scavenging
Chicken production system at farmer's level. Doctoral thesis. pp.160.

Edwards C A 1998
The use of earthworms in the breakdown and management of organic wastes. In C A Edwards (editor) 850 Earthworm Ecology: 327-376.
Boca Raton, FL: CRC Press.